1. Field
Exemplary embodiments of the present invention relate to a semiconductor design technology, and more particularly, to a semiconductor memory device capable of supporting a self-refresh operation.
2. Description of the Related Art
A unit memory cell of a typical Dynamic Random Access Memory (DRAM) device includes one transistor and one capacitor in which data is stored. The data stored in the capacitor formed over a semiconductor substrate may be damaged due to natural leakage. Therefore, a DRAM device has to perform a refresh operation periodically to recharge the memory cell with data. If the refresh operation is not performed stably, the data may be damaged, the read performance of the semiconductor memory device may be deteriorated, or the semiconductor memory device may malfunction.
The refresh operations are categorized into auto-refresh operations and self-refresh operations. The auto-refresh operation is performed while a semiconductor memory device performs a normal data input/output operation. The self-refresh operation is performed when the semiconductor memory device has not operated for a period of time, for example, while in standby mode or a power-saving mode. In short, the auto-refresh operation has to be performed while the semiconductor memory device is in the middle of performing a non al data input/output operation, the refresh operation is performed in response to an external refresh command inputted from an external source at a predetermined period. On the other hand, the self-refresh operation is performed while the semiconductor memory device is not performing any operation, and thus the refresh operation is performed at a predetermined period without receiving an external refresh command.
The refresh operation includes a series of processes of sensing and amplifying the data stored in the memory cells of a particular word line through an active operation, storing the data back in the memory cells of the word line, and pre-charging the word line. When the pre-charge operation is to be performed after the active operation begins is an important factor that needs to be considered in order to perform a stable refresh operation.